PICOLITER SAMPLE PREPARATION IN MALDI-TOF MS USING A MICROMACHINED SILICON FLOW-THROUGH DISPENSER

This paper presents a picoliter sample preparation technique utilizing the now-through principle, allowing on-line coupling of chromatograph ic systems to be made. The work was performed in order to investigate the characteristics and the physicochemical properties of the sample p reparation using typical mobile phase conditions from mu-CLC (column l iquid chromatography) separations, The device presented here is a pres sure pulse-driven dispenser, formed by two silicon structures processe d by conventional micromachining. The pressure pulse is generated in t he now-through channel by a piezoceramic element, Depending on the ori fice size, the droplets ejected range between 30 and 200 pL. The maxim um ejection frequency is 500 Hz, limited by resonances within the unit . A pyramid-shaped nozzle improves the directivity of the droplets sin ce it reduces the wetting of the orifice front surface area. The risk of particles sticking close to the orifice is also minimized. The anal yses of the deposited sample spots were carried out on a matrix-assist ed laser desorption/ionization time-of-night mass spectrometer with de layed extraction. It was possible to detect attomole amounts (159-248 amol) of various proteins (cytochrome c, ribonuclease A; lysozyme, and myoglobin) from a single droplet of matrix:analyte 1:1 (drop volume a pproximate to 110 pL). Additionally, it was found that sample enrichme nt could be carried out using multiple depositions on the same spot; i .e., 31 nM of insulin was easily detected when more than four depositi ons were made on the same spot, while no detection was possible withou t sample enrichment. Size optimization of the MALDI sample spot gave t arget zones of 100-500-mu m diameter that matched the size of the lase r focal point and resulted in a considerably increased sample throughp ut.